Velocity selector method for the separation of isotopes



Dec. 31, 1957 R. J. BRITTEN 2,813,507

VELOCITY SELECTOR METHOD FOR THE SEPARATION OF ISOTOPES meduarcn 22,194e 3S a j u NNN d U Y1 iw a m P "G l w k l may W10-Jay J N A u N g g,LH

:N Ill." l N w; 4 L; Q Q m. La' La m Wfw@ www' United States PatentVELOCITY SELECTOR METHOD FOR THE SEPARATION OF ISOTOPES Roy J. Britten,Washington, D. C., assignor to the United States of America asrepresented by the United States Atomic Energy Commission ApplicationMarch 22, 1946, Serial No. 656,467

3 Claims. (Cl. Z50-41.9)

This invention relates to a method and apparatus for separating andcollecting an enriched fraction of the isotope of a particular element;or for separating and collect-ing one kind of atom or molecule presentin a mass containing other atoms or molecules. This invention isparticularly directed to the production of a concentrated or enrichedfraction containing or consisting of uranium atoms of mass 235 by thetreatment of natural or other uranium which contains the uranium isotopeof mass 235 and also one or more other isotopes of uranium, chiey thatof mass 238.

One type of apparatus most frequently employed for accomplishing theseparation of isotopes or for obtaining a fraction of an elementenriched in its less abundant isotope, is the mass spectrometer, theoperation of this apparatus being based upon the principle that a beamcontaining two or more kinds of ions with equal charge and kineticenergy but of different mass will be resolved into separate orbitsaccording to the respective mass of the ions by a substantially uniformand constant magnetic eld through which the beam is caused to pass.There are very considerable diculties involved, however, in the use ofthis method, particularly that of maintaining a magnetic eld of suitablestrength, form and constancy. Furthermore, the degree of resolutionobtainable by the mass spectrometer requires that a relatively narrowbeam be utilized.

It is a principal object of this invention to overcome the above-noteddiiculties of the prior methods by developing a method and apparatus forseparating isotopes that dispenses with the use of a strong magnetic eldof predetermined form and constancy.

Moreover, when mass spectrometer is employed to produce uranium said tobe enriched as to the isotope 235 the content of the latter may beactually a very minor part of the whole because the resolutionobtainable is not as great as desired and also because in naturaluranium the 235 isotope is only about 0.71% of the element (the 238isotope counting for about 99.28% and the 234 isotope for about 0.006%).Accordingly, it is a further object of this inventionto provide a methodand apparatus for separating isotopes that hashigh resolving power.

It is a still further object of this invention to provide a method andapparatus for separating or collecting an enriched fraction of aparticular isotope of a polyisotopic substance by selectivelyaccelerating the ionized constituents present in a beam of thepolyisotopic substance that are of uniform kinetic energy, the saidacceleration being applied intermittently and at spaced p oints alongthe beam and in a direction normal to the direction of propagation ofsaid uniform energy beam whereby a transverse displacement of theisotopic constituents of diiferent mass is obtained.

Other more specific objects of this invention will become apparent as itis described in detail in connection with the drawing accompanying thisspecification wherein:

Patented Dec. 31, 1957 Fig. 1 is a schematic illustration in crosssection of a preferred embodiment of the invention;

Fig. 2 is a cross-sectional end view, the section being taken on line 22, of Fig. l;

Fig. 3 is a plot of a curve representing the ion current densitydistribution at the collector for the uranium ion of mass 238; and

Fig. 4 is a plot of a curve representing the ion current densitydistribution at the collector for the uranium ion of mass 235.

Referring now to the drawings, Fig. l is a schematic representation ofthe assembled mass separator and diagrammatically indicates the methodof applying various potentials to the elements of the assembly in orderto accomplish the isotope separation. More particularly, the apparatusillustrated 'in Fig. l comprises an elongated and highly evacuatedchamber 10 supporting an ion source 11 and a collector plate 12 at itsopposite ends. The ion source housing 13 is supported on the end plate14 of the chamber 10 which end plate is insulated from the main chamberby the heavy insulating collar 15; so that the source housing may bemaintained a high positive potential with respect to the remainder ofthe chamber. The ion source housing 13 is partially broken away in thedrawings to show the electrodes 16 and 17 of the arc chamber, betweenwhich is established an arc discharge in an atmosphere of a readilyvaporizable uranium compound, uranium tetrachloride for example. Theelectrodes illustrated comprise a lamentary electron emissive cathode 16and a plate-like anode 17. The ions produced in the are establishedbetween the two electrodes by the application thereto of a directcurrent Voltage illustrated at 18, are withdrawn from the centralslit-like opening in front of the end plate 20; and are accelerated in abeam (the direction of which is along the abscissa designated by theletter X in the drawing) by the difference in the potentials appliedbetween the source screen'assembly 21 and the source housing 13. Thesource screen 21 is shown supported in the chamber 10 by a collar 22 soas to be at the same potential as the housing. By reason of thediierence in direct current potential 23 applied between the housing ofthe ion source 13 and the ion accelerating screen 21 the ions of likecharge that are generated in the are chamber are withdrawn through theopening 19 in the formof a beam and are given a uniform kinetic energyby falling through' this xed potential.

The accelerating electrode will therefore impart substantially equalkinetic energy to all the ions as they leave the source in the directionalong the abscissa O-X. In this way there will be obtained a beam ofions constituting U1235 and U+238 of substantially uniform kineticenergy.

The ion beam thus produced enters the space between the rst pair ofplates 24 and 25 to which is applied the radio frequency alternatingcurrentvoltage indicated at 36 in the drawing. There is also shownsupported in the chamber 10 a plurality of similar plates 264-27, 28-29,30-31, 32-33 and 34--35, each of the said plates being of the samedimensions and having a uniform spacing a/2 between their centers. Theaggregate of these pairs of plates is shown supported within the chamber10 by means of the insulating block members 37, a respective one ofwhich is secured to each plate and to the housing so that the plates arealigned symmetrically with respect to the direction of the ion beam asit leaves the source housing 13. Each of the plates of the saidrespective pairs of plates is also connected to one side of the sourceof the radio frequency voltage 36 by the conductors 38 which are shownin the drawing as extending through the center of each insulating block37 and through an insulating and sealing bushing supported in thehousing and indicated schematically in the drawing at 39.

The mode of connecting the pairs of plates as indicated by the drawingsis such that the potential of each plate is 180 out of phase with thepotential of a similarly positioned and directly adjacent plates; andalso 180 out of phase with the plate lying diametrically opposite.

The amplitude and the frequency of the radio kfrequency voltage and thechoice of spacing between successive pairs of plates, and the number ofpairs of plates are determined by the particular separation which theapparatus is required to accomplish as will be apparent later from aconsideration of an illustrated example. The ion current collector isshown supported by the end of the chamber 10 spaced from vthe last pairof deector plates 341-35. The showing of the collector is schematiconly, it being understood that the collector may be connected in circuitto provide a suitable deceleration of the ions prior to collection andmay be modified to inhibit electron emission as the result ofion'bombardment or otherwise in accordance with the teachings oftheprior art.

The operation of the apparatus is described hereinafter for carrying outthe separation or enrichment of the rare uranium isotope of weight -235from the much more abundant isotope of weight 238, it being understood,however, that the invention is not so limited and may be applied equallywell to the separation or enrichment of one or any number of isotopes ofother elements; and also to the separation or enrichment of one kind ofatom or molecule present in a mixture of other atoms or molecules ofdiiferent mass.

The energy imparted to the ions by the potential difference between theion source housing'13 and the accelerating electrode 21 and the distancea/Z'between the mid points of the adjacent pairs of plates are sointerrelated that the time necessary for the light ion to traverse thedistance of a/Z equal to the spacing between the mid points ofsuccessive plates should be equal to one-half the period of thealternating voltage. Since the heavy ion Ui'238 will have the samekinetic energy as the light ion U+235 the former will lag behind thelatter asthey travel down the chamber 10; i. e., the heavy ion thatenters the space between the pairs of plates at the same time asA thelighter ions will emerge from'the space after the aforementioned lighterions. Therefore the number of pairs of plates can-berselected so thatthelight ions will vhave traversed the entire length of Ithel space betweensuccessive pairs of plates in the period of time required for the' heavyion-s starting at the same instant as the light ions to traverse` theentire. lengthof the successive pairs` of plates except'formthe lastytwo pairs. -That-is, the heavy ions will lag the light ion by ardistanceequal tothe-length of travel of the lightfion in the-time requiredforonefull cycle of the radio frequency voltage 36.

The net effect of this selection of the number ofpairs of plates is thatthe heavy ion will emerge from the space between the last pair ofdeecting plates (S4-35) moving substantially in the same direction vthatit had before it entered the space between the first pair of deliectingplates (2425). The light ionon the contrary will emerge from the lastpair of deecting plates (S4-35) with a transverse displacement in a`direction :L- oy the magnitude and sign of which `Will depend upon thecycle ofthe alternating voltage at the instant this light ion enteredthe space between the rst pair lof plates 24-25. i Thus the heavy ionswill form a deposit on the central region of the collector plate 12, thewidth of which will `besubstantially equal to that of the original ionbeam (neglecting the mutual repulsion between the charged particles). Anidea as to the distribution of this-heavy ion on the collector may behad by considering Fig..u3 whichrepresents graphically thel variations.in ion.` .current density -for v the U238+ ion with the `displacementfrom the.y center oft-the.

collector plate 12.

Cal

The light ion on the other hand form a 'widely spread deposit on thecollector plates only part of which will be superimposed upon thedeposit of the heavy ions. Thus a large fraction of the deposit of thelight ions will be substantially free of the heavy isotope. An idea ofthe distribution of the deposit of the light ion on the collector may behad by considering Fig. 4 whichl is a plot, derived from numericalcalculation, of the ion current density distribution for the lightisotope with distance from the center of the collector plate.' Th'evalues for the ordinate in Figs. 3 and 4 are obviously unequal by atleast a factor of 500.

In order that the above description of the operation may be more clearlyunderstood and the invention readily practiced the following preciseillustration is presented:

Assuming a value for the ion accelerating voltage 23 equal to 1000 voltsand a spacing between the centers of adjacent plates (zz/2) being set at0.5 centimeter the velocity imparted by the accelerating electrode to'arsinglycharged U235ion, the type which' it is desired to concentrate,may be obtained from the equation:

whereinmv is the mass of the U235-isotope inl grams, v is itsl velocityin'centimeters per second, eis the elec tronic charge (e. s. u.) and Vis the potential drop in volts. By substituting the known values of the'electronic charge andthe mass of the U235. atom into they above equationand alsoby employing the aforementioned value of 1000 volts for theypotential drop, the Velocity ofthe U235 ion is found to be 2.84)(106cm/sec. The velocity of the U239 ion canl also be derived by theabove equation; or

otherwise by multiplying the expression for the velocity ofV the U235ion by the square rootof -the ratio of the masses of the 235to the 238ionthus:

The presence of the square root in the above expression isv due ofcourse to the fact that two equally-charged particles of=unequalmassboth startingfrom rest and drop ping through the same potentialv willgain velocity in versely proportionally to the square root of theirrespectivemasses.A

Since itis desired-that the time necessary for the light ion totraversethe' distance between the'centers of ad be equalfto 1A thevperiodof theradio frequency voltage it is apparent that, the frequency of` this4voltage must be equal to 2'.84.megacyc1es per second. Asgthe ion beamenters theyspace between the deecting plates traveling in a directionsubstantially along orl parallel to the abscissa in Fig.` 1, the'component of the velocity it receives' in the direction along theabscissa ,willbe practically constant for any ion light'or heavy asit'proceedsV down the tube to the collector plate since the field"between pairs of platesis perpendicular to this direction. On the othervof detlector. plates must be such that the light ionwill lead the heavyion by a distance equal to twice the'spacebetween-the centers-of Vthedeflectingplates or the distance traveledby -the light ion in onecomplete cycle of they Accordingly, the total lengthl radio frequencyvoltage. oftheset. of deecting platesmay: bedetermined` from theequationsi wherein af is the ratio of the velocity of the light ion to thevelocity of the heavy ion and a is twice the distance between centers ofa pair of adjacent plates. For the values stipulated above the length ofthe set of dellection plates is found to be equal to 158 cm. The spacingbetween the centers of the adjacent pairs of deiiector plates wasstipulated to be equal to 0.5 cm., therefore the number of pairs ofplates is equal to the length of the set of deflector plates divided bythe spacing between the centers of adjacent pairs of deilector plates orwherein: x is the distance of the particular point for which the fieldstrength is being calculated, the said distance being computed from theorigin o in Fig. 1 which is at a distance a/ 4 from the center of thefirst pair of deecting plates in the direction opposite the direction oftravel of the ion beam. In the above equation, qb represents the phaseof the deilecting voltage at the instant the particular ion arrives atthe origin; at the time t=0 and at a g phase angle =0, this ion willpass the center of the rst pair of deilecting plates at a maximumvoltage. In the above equation w=21rf wherein i is the frequency of thedeflecting Voltage.

Considering now the particular ion that remains in phase with thedeecting voltage that is the ion U235. The distance X traveled by thelight ion, at which the field strength is to be determined by the aboveequation is equal to the velocity of this particular ion multiplied bythe time t, i. e.,

The frequency of the deflecting voltage is as noted above given by theequation:

and since w=2v/ a 8 therefore,

2 Tg2-I1 wi 9 Now the acceleration for the ionized particle whichremains in phase with the voltage (i. e., the uranium ion of mass 235)in the direction `of the Y axis is given by the following equation:

d2y/dt2=A(Cos g5 Sin2 wt-l-Sin o Sin wt Cos wt 10 wherein A is themaximum acceleration; which can be determined by the following equation:

wherein e is the charge of an electron (esiz) and ML is the mass ingrams of the light ion U235+ and Eo is the peak field strength in volts.Integrating this equation, and solving for the constants Iofintegration, it is found 4that in the particular case requiring a largenumber of plates, deflection in the Y direction of the light ion at 6the nal deccting plate (making a few approximations), may be written asfollows: c,

A La@ For the heavy ion U233+ which does not remain in phase with thedeilecting voltage the distance X from the origin that this ion hastraveled in the same time. interval t may be given in the followingequation:

transverse to the direction of propagation, i. e., the Y direction, maybe determined by the following equation:

d2y/dt2=B(Cos 4a Sin wt Sin awt-l-Sin qb Cos wt Sin awt) 15 Integrating,solving for the constants of integration and making an approximation,the equation may be reduced to the following:

At the final deflecting plate which is at a distance X, from the originO equal to Sin qS Sin (ae-1) the dellection in the Y direction given theheavy ion U238+ may be computed by the following equation:

B 41rd Sill da yf-zwua-iy +1 17 In the above equations B is equal to themaximum ac-` celeration given the heavy ion, and may be found bymultiplying A by the ratio of the masses thus B A MH 18 At the sameposition at the distance X, from the origin, the deection of the lightion in the Y direction may be ycomputed from the following equation:

For the particular data assumed above the deection of the heavy ion isfound to be equal to .503 sin p and the deection of the lighter ion atthe distnce Xf from the origin (for the data assumed above) is equal to1.59 cos fp.

Since it is proposed in a specific embodiment of this invention to placethe collector at a distance from the last pair of plates equal to thetotal length of the pair of plates the slope or angle of the heavy andlight ion leaving the last pair yof plates should also be determined.This may be done by taking the first derivation with respect to thedistance X along the abscissaof the above equations for thedisplacement, thus:

1r(a-l)3 Sin Sin qS and in the specific case X=Xf ffy/dx (Heavy)=.oo0126sin p 22 dy/dx (Light) :0.020 Cos qi 23 Utilizing the above equations,calculationsshow that the maximum spread of the heavy ion is` 1.0l cm.,the maximum spread of the light ion isr m53 cm., and that more than 50%of the light ions strikethe collector plate at a distance plus or minus3.0 cm. eitherside of its center. It is apparent that with this highdegree of resolution one can obtain a practically pure concentration ofthe UZ isotope.

It might be more practical, however, to utilize a smaller deflection ora more diffuse beam and not attempt to obtain the nearly pure lightisotope in the first stage. Accordingly, there is set forth belowanother specifi-c example of a separator as follows:

Number of detiector plates 316 Ion accelerating voltage 20,000 voltsFrequency of deiiecting tield 2.84 megacycles. Plate spacing (along thebeam) (a/2)==2.23 cm. Length of set plates a/ot-1 707 cm.

Distance from the end of deflector set to the collector 300 cm. Peakdeflecting field 5 volts per cm. Undetiectedwidth of beam in the y-direction at collector 20 cm. Maximum spread of heavy ions plus orminus 15 cm. Maximum spread of light ions plus or minus cm.

More than of the light ions fall outside of I2O cm.

If vapors other than pure uranium (uranium tetrachloride forexample) areutilized to sustain the arc in the ion source, ions other than thosedealt with in the above mathematics Willbe produced. For example,combined ions of uranium tetrachloride may be produced, and would haveamass to charge ratio different by the mass of the light chlorine atom ofthe compound. Accordingly, these ions will' lag behind, the light 235+ion say by N cycles. During the part of the travel of this combined ionin which the lag is building up to an even number of cycles, it will getno resultant displacement as deviation in direction. (It should be clearfrom the foregoing that the heavy (238) ion although lit receives nodeviation in direction may be displaced in the beam, an amount dependingupon vthe phase of the A. C. when it enters the first pair of defiectingplates.) During any odd cycle that is left over the combined ion may geta displacement in the y direction which is equal to' l/N times thedisplacement of the 238+ ion. During any portion of a cycle that is leftover the combined ion (U235C135Jf) may get a deviation in directionwhich is a small percentage of the deviationlof the 235+ ion. In thecase of the (U238C135+) ion ofmass 273, the total lag will be 12.3cycles behind the 235+ ion. The deviation in direction will be less thanV40 of thatl of the 235+ ion. Thus, it will be apparent that other typesof lions only add to the current in the main undeliected`- beam and.vtherefore, may increase the number ofsscatteredions andthe space chargeeffects, but do not otherwiseinterfere.

- Withirespect to the deviation andlateraltranslation or displacement ofthe ions it might be noted that the ion of the isotope getting; thegreater ldeviation (.235?.r in theY casev assumed). receives,v itsmaximum. dev1at1on 1f it ar-y rives at themidpoint of the. firstdetiector plates in phase with. the maximum` of .the applied. voltage.The ion of the isotope which. is little deviated (238) gets its greatestlateral translation or displacement when it arrives at the firstdeiiectorv plate when` the voltage is zero. It is apparent then, that atsome instant lthe distance. between the twoisotopic ionbeams at thecollector is greater than the average distance. Accordingly, it might bedesirable to make use of this phenomenon by adding an additional set ofplates with its proper phase and magnitude of the applied voltage, itmay be desirable also to use a fraction of the number of deflectorplates originally described with this later deector.

lt may be noted that near the deecting plates there is an alternatingvcomponent of the field parallel to the direction of travel of the ionbeam. It this component is large enough to be important its effect willbe to bunch the ions so that they come more into phase with the maximumdeflecting voltage.

The above specific examples of this invention have been set forth withthe understanding that numerous modifications in the construction andarrangement of parts may be made. For example, the deflecting platesutilized to establish an electric field normal to the direction ofpropagation of the ion beam may simply comprise a pair of parallelwires, spaced apart and set at right angles to the directionv of thebeam; Moreover, the invention is not limited tol the utilization ofperiodic electric field means for effecting the desired separation ofthe isotopic constituents of the beam, since there are yother meansknown to deflect moving charge particles, i. e., a magnetic field. Onemethod of utilizing a magnetic rather than an electric field toproducethe desired deflection of an ion of a particular mass to charge ratio inthe beam would be to utilize a plurality of tank coils of an R. F.oscillator and to space these tank coils along the beam to set up timedand spaced magnetic fields in a direction transverse to the direction ofpropagation of the beam. The frequency spacing and phase relation ofthese magnetic fields could be set to deflect an ion of predeterminedvelocity and to produce no appreciable resultant deection on ions ofother velocities. It is clear therefore in view of the foregoing thatmany variations may be made in the [particular structure disclosedwithout departing from the-scope of: this invention as set forth in theappendedV set of claims.

The present apparatus can be used without change for the separation ofisotopes of elements other than uranium.

I claim:

1. An apparatus for separating ions in accordance with their mass tocharge ratio comprising in combination an elongated evacuated chamber,means for propagating aA beam of said ions at uniform kinetic energythrough said chamber in the direction of its elongation wherebysimilarly charged ions will have a velocity inversely proportional tothe square root of their respective masses, a plurality of pairs ofdeflecting plates positioned longitudinally at spaced points along saidbeam for subjecting said beam of ions to a series of spaced consecutivealternating electric fields having a component transverse to thedirection of propagation of said beam, a source of high frequencyalternating current, successive pairs of said plates being connected tosaid' source simultaneously in opposite phase to each other, whereby aparticular ion ofl predetermined mass to charge ratio will be deflectedin consecutive opposite directions at each of the alternating fields ofsaid series, Ithe number of said separate iields of said seriesbeing-equal to-Z/ 1*1 wherein a is the ratio of the velocity of the ionof predetermined mass to charge ratio to the velocity of another ionconstituent of said beam having` agreater mass to charge ratio, and asingle collectormeans: for simultaneously collecting; theseparatedlions,rsaid'means consisting of a collector platewithin and adjacent one endof the elongated evacuated chamber, said collector being arrangedtransversely of the beam at a point rectilinearly spaced from the finalpair of deecting plates in the direction of the beam, said collectorplate providing separate zones for simultaneously collecting both thedeliected and undeiiected ions.

2. An apparatus for separating or collecting an enriched fraction of aparticular isotope of a polyisotopic substance from its other isotopicconstituent comprising an evacuated chamber containing means forionizing a vapor of said polyisotopic substance, means for applying apredetermined accelerating potential to said ions of like charge wherebyto form a longitudinal beam of ions having uniform kinetic energy, aplurality of pairs of deflecting plates positioned longitudinally atspaced points along said beam, means for applying an alternating voltageto each of said pair of plates so that the polarity of the platespositioned longitudinally and diametrically adjacent a particular platewill be 180 out of phase with the potential of the said particularplate, and means for simultaneously collecting the separated ionscomprising a collector plate within and adjacent one end of theevacuated chamber, said collector plate being arranged trans- Versely ofthe beam at a point rectilinearly spaced from the final pair ofdeiiecting plates in the direction of the beam, said collector plateproviding separate zones for simultaneously collecting both thedeflected and the undeected ions.

3. An apparatus for separating or collecting an enriched fraction of aparticular isotope of a polyisotopic substance from its other isotopicconstituents comprising in combination an elongated evacuated chambersupporting at one end thereof, means for producing gaseous ions of saidpolyisotopic substance, means for imparting a uniform kinetic energy tosaid ions to propagate said ions in 10 the form of a beam throughoutsaid chamber in the direction of the elongation whereby said ions oflike charge will acquire velocities inversely proportional to the squareroot of their respective masses, a plurality of pairs of deilectingplates positioned longitudinally at spaced points along said beam forimpressing at points spaced along the path of said beam an alternatingelectric eld having a component transverse to the direction ofpropagation of said beam to cause the ions to be deflected transverselyof said beam, a source of high frequency alternating current, successivepairs of said plates being connected to said source simultaneously inopposite phase to each other, the number of said spaced points beingequal to 2/a-1 wherein a is the square root of the ratio of the mass ofthe heavy isotope to the mass of the light isotope, whereby the ion ofthe heavy isotope will emerge substantially undeected in traversing thedistance along which said fields are impressed, means for simultaneouslycollecting the separated ions comprising a collector plate within andadjacent one end of the evacuated chamber, said collector plate beingarranged transversely of the beam at a point rectilinearly spaced fromthe iinal pair of deflecting plates in the direction of the beam, saidcollector plate providing separate zones for simultaneously collectingboth the deflected and Ithc undeected ions.

References Cited in the file of this patent UNITED STATES PATENTSWinkler Oct. 23, 1945 OTHER REFERENCES

1. AN APPARATUS FOR SEPARATING IONS IN ACCORDANCE WITH THEIR MASS TOCHARGE RATIO COMPRISING IN COMBINATION AN ELONGATED EVACUATED CHAMBER,MEANS FOR PROPAGATING A BEAM OF SAID IONS AT UNIFORM KINETIC ENERGYTHROUGH SAID CHAMBER IN THE DIRECTION OF ITS ELONGATION WHERBY SIMILARLYCHARGED IONS WILL HAVE A VELOCITY INVERSELY PROPORTIONAL TO THE SQUAREROOT OF THEIR RESPECTIVE MASSES, A PLURALITY OF PAIRS OF DEFLECTINGPLATES POSITIONED LONGITUDINALLY AT SPACED POINTS ALONG SAID BEAM FORSUBJECTING SAID BEAM OF IONS TO A SERIES OF SPACED CONSECUTIVEALTERNATING ELECTRIC FIELDS HAVING A COMPONENT TRANSVERSE TO THEDIRECTION OF PROPAGATION OF SAID BEAM, A SOURCE OF HIGH FREQUENCYALTERNATING CURRENT, SUCCESSIVE PAIRS OF SAID PLATES BEING CONNECTED TOSAID SOURCE SIMULTANEOUSLY IN OPPOSITE PHASE TO EACH OTHER, WHEREBY APARTICULAR ION OF PREDETERMINED MASS TO CHARGE RATIO WILL BE DEFLECTED